This invention relates to a process for producing a heating element which is to be arranged in a plastic part and used to weld the plastic part to a plastic pipe, wherein the heating element comprises a heating wire wound as bifilar spiral with radially outwardly extending wire ends through which current can be supplied to the heating element. The invention relates furthermore to an apparatus for implementing the process of the invention.
A heating element which is used to weld a saddle piece or a tapping fitting of plastic to a pipe likewise consisting of plastic is disclosed in European Patent Application No. EP 069,020. The heating element contains a thin plate of plastic, the one surface of which has a spiral groove. Inserted in this spiral groove is the heating wire, which has a round cross section. The plate has two diametrically opposite projections with the ends of the heating wire, to which electric current is fed for welding from a welding appliance. One end of the heating wire or of the welding winding is led from the center of the plate radially outward to the assigned projection. The plate is provided with a strap which, after insertion of the welding winding into the spiral groove is placed over the welding winding, the inner end of the welding winding then being led radially outward via the strap to the projection. The strap is intended to prevent a short circuit during welding. The production of this known heating element involves significant expense, particularly with regard to the aforementioned strap. During welding, additional difficulties may arise due to excessive local heating in the vicinity of the strap or of the radial leadout of the wire end. An increased amount of heat, in comparison with the other regions of the welding winding, is namely supplied in this region through the radially outwardly extending wire. There is a risk of void formation and reduced welding quality. Furthermore, since the spiral welding winding is arranged in the groove of the plastic plate, the heating wire cannot bear directly against the surface of the pipe to be welded. Rather, the welding winding is spaced a distance, corresponding to the base thickness, from the pipe surface, and a correspondingly increased heating power must be supplied.
An electrically weldable shaped tapping piece of thermoplastic material is disclosed in Swiss Patent No. CH 528,697. This shaped tapping piece is produced by injection molding, with a sheathed resistance wire being placed into the injection mold before it is closed. This sheathed resistance wire has first been cast into a plate likewise consisting of thermoplastic material. In this plate, the windings of the sheathed resistance wire lie closely adjacent one another. The resistant wire may be wound as a single spiral or as a bifilar spiral, the spacing of the individual windings being defined by the wall thickness of the plastic sheathing. The winding spacing cannot be readily changed or optimized with regard to the necessary welding performance. A person skilled in the art is not given any other indication as to how to predetermine the spacing of the windings from one another other than by appropriate material thickness of the sheathing. Furthermore, due to the sheathing, the resistance wire is spaced a considerable distance from the surface of the pipe to be welded, so that a relatively high use of energy is necessary.
Finally, a shaped tapping piece with a heating wire mat is known from German Utility Model No. 85 21 529. To produce this heating wire mat, the heating wire provided with a plastic sheathing is arranged in zig-zag form, the individual parts of the heating wire mat lying closely adjacent one another. Subsequently, the closely adjacent parts of the heating wire are welded to one another. This welding can only be performed with plastic-sheathed heating wire. Here again the spacing of the individual regions of the heating wire depends on the wall thickness of the sheathing.